请用python语言实现以下问题并给出解题思路：As a example, consider the text THE CAT IN THE HAT. In this text, the letter T and the space character both occur with the highest frequency, so they will clearly have the shortest encoding bit patterns in an optimal encoding. The letters C, I and N only occur once, however, so they will have the longest codes. There are many possible sets of prefix-free variable-length bit patterns that would yield the optimal encoding, that is, that would allow the text to be encoded in the fewest number of bits. One such optimal encoding is to encode spaces with 00, A with 100, C with 1110, E with 1111, H with 110, I with 1010, N with 1011 and T with 01. The optimal encoding therefore requires only 51 bits compared to the 144 that would be necessary to encode the message with 8-bit ASCII encoding, a compression ratio of 2.8 to 1. Input The input file will contain a list of text strings, one per line. The text strings will consist only of uppercase alphanumeric characters and underscores (which are used in place of spaces). The end of the input will be signalled by a line containing only the word END as the text string. This line should not be processed. Output For each text string in the input, output the length in bits of the 8-bit ASCII encoding, the length in bits of an optimal prefix-free variable-length encoding, and the compression ratio accurate to one decimal point.

题目描述：

给定一个由大写字母，数字和下划线组成的文本字符串，将其用 8 位 ASCII 编码和最优前缀自由可变长度编码进行编码，并计算压缩比。

解题思路：

首先，需要对输入的文本字符串进行字符频率分析，得到每个字符出现的次数。根据字符频率，可以构建霍夫曼树并生成最优前缀自由可变长度编码。最后，计算 8 位 ASCII 编码和最优编码的长度，并计算压缩比。

具体步骤如下：

1.读入文本字符串，统计每个字符出现的次数。

2.根据字符频率构建霍夫曼树，并生成最优前缀自由可变长度编码。

3.计算文本字符串用 8 位 ASCII 编码和最优编码的长度。

4.计算压缩比并输出结果。

代码实现如下：